The present invention relates to an A/D conversion apparatus which converts an analog value into a digital value, and a solid-state image sensing apparatus including the A/D conversion apparatus.
A solid-state image sensing apparatus comprises a plurality of photodetectors arrayed one- or two-dimensionally, and an integrating circuit which converts a current output from each photodetector into a voltage. In the solid-state image sensing apparatus, currents of values corresponding to incident light intensities are output from the respective photodetectors. Voltages of values corresponding to the current values are output from the integrating circuit. An incident light intensity distribution is obtained based on the voltage values to sense an image.
Some solid-state image sensing apparatuses further comprise an A/D conversion circuit which converts a voltage value (analog value) output from the integrating circuit into a digital value. In this case, the incident light intensity is obtained as a digital value and can undergo image processing by a computer or the like.
In general, demands have arisen for increasing the A/D conversion processing speed including that for the solid-state image sensing apparatus. For example, in an A/D conversion apparatus described in reference 1 (Yuh-Min Lee, et al., xe2x80x9cA 13-b 2.5-MHz Self-Calibrated Pipelined A/D Converter In 3- xcexcm CMOSxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 26, No. 4, pp. 628-636 (1991)), a plurality of sets each made up of a sample-and-hold circuit, A/D conversion circuit, D/A conversion circuit, and amplifier circuit are cascade-connected.
In a given set of the A/D conversion apparatus, an analog value output from the sample-and-hold circuit is converted into a digital value by the A/D conversion circuit, and the digital value is converted into an analog value by the D/A conversion circuit. The difference between the analog values respectively output from the sample-and-hold circuit and D/A conversion circuit is amplified by the amplifier circuit. The amplified difference (analog value) is input to the sample-and-hold circuit of the next set. This A/D conversion apparatus increases the speed by pipeline processing.
In an A/D conversion apparatus described in reference 2 (Steven Decker, et al., xe2x80x9cA 256xc3x97256 CMOS Imaging Array with Wide Dynamic Range Pixels and Column-Parallel Digital Outputxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 33, No. 12, pp. 2081-2091 (1998)), two sets each made up of a comparison circuit (1-bit A/D conversion circuit), amplifier circuit (amplification factor: 2), and addition circuit are arranged in a loop.
In a given set of this A/D conversion apparatus, an input analog value is converted into a 1-bit digital value by the comparison circuit, and the digital value is amplified twice by the amplifier circuit. A predetermined analog value is added to or subtracted from the analog value output from the amplifier circuit in accordance with a 1-bit digital value output from the comparison circuit. The resultant value is output to the next set.
The A/D conversion apparatus realizes a small circuit scale as a whole by downsizing the circuit scales of the comparison circuit, amplifier circuit, and addition circuit of each set and repetitively operating the two sets arranged in a loop.
The A/D conversion apparatus described in reference 1 achieves a high speed by the pipeline arrangement. As the number of bits of a digital value becomes larger, the circuit scale becomes larger. The A/D conversion apparatus must be equipped not only with the A/D conversion circuit but also with the D/A conversion circuit, which increases the circuit scale. Further, the precision of the obtained digital value is influenced by a D/A conversion error.
The A/D conversion apparatus described in reference 2 attains a small circuit scale as a whole by downsizing the scale of each circuit of each set and arranging the two sets in a loop. However, a signal passes through the number of sets of comparison circuits, amplifier circuits, and addition circuits corresponding to the number of bits of a digital value. The precision of the obtained digital value is greatly influenced by an error of each circuit of each set. If the operation characteristics of the comparison circuit, amplifier circuit, and addition circuit mismatch in each set, this mismatch also influences the precision of the obtained digital value.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide an A/D conversion apparatus with a small circuit scale and a small A/D conversion error, and a solid-state image sensing apparatus including the A/D conversion apparatus.
An A/D conversion apparatus according to the present invention is characterized in that a difference between a first analog value input to an A/D conversion circuit and a second analog value corresponding to a digital value output from the A/D conversion circuit is amplified, and the amplified difference is fed back as a third analog value to an input terminal of the A/D conversion circuit after input of the first analog value.
The A/D conversion apparatus is characterized by comprising a connection switching circuit which selectively inputs either of the first and third analog values to the A/D conversion circuit.
In other words, the A/D conversion apparatus comprises (A) a connection switching circuit which has first, second, and third terminals and connects either the first or second terminal to the third terminal, (B) an A/D conversion circuit which receives an analog value from the third terminal of the connection switching circuit, converts the analog value to a digital value, and outputs the digital value, (C) an amplifier circuit which amplifies a difference between an analog value corresponding to the digital value output from the A/D conversion circuit and the analog value input to the A/D conversion circuit, and outputs the amplified analog value to the second terminal of the connection switching circuit, and (D) control means for controlling switching of connection between the first, second, and third terminals of the connection switching circuit.
In the A/D conversion apparatus, an analog value input to the first terminal of the connection switching circuit is input to the A/D conversion circuit via the third terminal under the control of the control means. The analog value is converted into a digital value by the A/D conversion circuit, and the digital value is output from the A/D conversion circuit.
To obtain a digital value of a larger number of bits, the difference between an analog value corresponding to the digital value output from the A/D conversion circuit and an analog value input to the A/D conversion circuit is amplified by the amplifier circuit. The amplified analog value is input to the A/D conversion circuit via the second and third terminals of the connection switching circuit. The analog value is converted into a digital value (corresponding to lower bits than a previously output digital value) by the A/D conversion circuit. The digital value is output from the A/D conversion circuit. This operation may be repeated.
The A/D conversion apparatus according to the present invention is characterized in that the A/D conversion circuit outputs the digital value of n bits (n is an integer of 1 or more), and the amplifier circuit amplifies the difference by 2n. In this case, A/D conversion processing is most efficiently performed.
A solid-state image sensing apparatus which senses an object is characterized by comprising (A) a photodetector which outputs a current of a value corresponding to an incident light intensity, (B) an integrating circuit which receives and integrates the current output from the photodetector, and outputs a voltage of a value corresponding to the current value, and (C) the A/D conversion apparatus which receives the voltage output from the integrating circuit and converts the voltage value into a digital value.
According to the solid-state image sensing apparatus, a current of a value corresponding to the incident light intensity in the photodetector is output from the photodetector and integrated by the integrating circuit. A voltage of a value corresponding to the current value is output from the integrating circuit. The voltage value (analog value) output from the integrating circuit is converted into a digital value by the A/D conversion apparatus.